Fuel cell structures comprising a fuel separator plate, an oxidant separator plate and a membrane electrode assembly (MEA) sandwiched between the plates are well known in the art. In fuel cell structures of the solid polymer electrolyte type, the MEA typically comprises a solid polymer electrolyte membrane sandwiched between two porous electrically conductive electrodes, one forming the anode, the other one forming the cathode. Catalysts, e.g. platinum, are disposed on the interface of the membrane and the electrodes.
A fuel gas, for example hydrogen, is supplied to the anode while an oxidant gas such as air (containing oxygen) or oxygen is supplied on the cathode side. The hydrogen will move across the porous anode and will be converted to protons and electrons on the anode catalyst. The protons are moved towards the cathode via the humidified polymer electrolyte. The oxygen moves through the porous cathode and reacts with the protons which have traversed the membrane to form water. The electrons which are generated are led through an external circuit. The current thus generated can directly by used as electric energy. The separator plates are made out of an electrically conductive material and act as current collectors. At the interface between the MEA and the separator plates, one or more fluid flow paths defined by channels formed in the plate direct the reactant fluids to the electrodes while leading the fluid from a supply port to a discharge port.
Typically, a plurality of fuel cell structures is stacked to form a fuel cell stack. The oxidant separator plate of one fuel cell structure then sits back to back with the oxidant separator plate of the neighbouring fuel cell structure, with a cooling flow path not being described herein between the two plates, such that electric current can flow from one fuel cell structure to the other and thus through the entire stack.
The separator plates further provide a fluid barrier between adjacent fuel cell structures so as to keep reactant fluid supplied to the anode of one cell from contaminating reactant fluid supplied to the cathode of another cell.
The arrangement of the fluid flow paths on the separator plate, i.e. the design of the so-called flow-field is crucial for the performance of a fuel cell. One important point is the uniformity of the reaction. While being led from the supply port to the discharge port, the reactant fluids are consumed on the electrode surfaces. The number of reactive molecules per area unit will thus decrease towards the outlet. It is desirable, however, to maintain a constant pressure and a constant flow speed over the complete flow path in order to obtain a uniform reaction distribution across the whole plate.